1. Field of the Invention
The present invention relates to a parallel-type hybrid car and particularly relates to a control apparatus for a hybrid car, which uses an electric double layer capacitor and which can provide a braking force for the car by charging the electric double layer capacitor by regeneration by the motor.
2. Background Art
Electric cars are currently under development in order to reduce the discharge of exhaust gas into the atmosphere. However, the cruising range of electric cars driven only by a motor is limited by the capacity of the battery which stores the electric energy. If sufficient cruising distance is desired, very large batteries are required, which seriously degrades the travelling performance of the car.
Hence, hybrid cars have becomes the subject of intensive development, because it is possible for hybrid cars to provide a large cruising range and good driving performance by using both an engine driven by the combustion of fossil fuel and a motor activated by a comparatively small battery.
Parallel-type hybrid cars, one type of hybrid car, realize good driving performance, reduced emission of exhaust gas, and reduced fuel consumption by switching the driving modes from engine drive mode to motor drive mode according to the driving conditions.
That is, hybrid cars travel using the engine only, when travelling using the engine is efficient, and the motor assists the engine or the motor drives the car without the engine when the travelling conditions are not conductive to efficient using the engine, such as when accelerating or when travelling slowly.
In addition, when braking the car, the motor is operated as a generator for recovering the kinetic energy of the car, and energy loss can be prevented and the fuel consumption can be reduced.
That is, when braking the car, the motor is rotated by the kinetic energy of the car, and the electric power generated by rotation of the motor is charged into the battery, so that the energy made available by braking can be recovered.
However, when the charging of the battery continues and the amount of charge exceeds the maximum charge limit of the battery and causes overcharging, the battery may be degraded.
In order prevent such overcharging of the battery, charging should be terminated when the battery is in the fully charged state. However, when regenerative braking is terminated as the car is braking, the braking force is lost, which not only reduces the feeling of drivability, but also imposes a burden on the brake mechanism, for example, on a long downhill stretch, because frequent braking operations are required.
In order to avoid such problems, a technique is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 10-23603, in which a braking force is obtained by use of engine braking by connecting the engine and the drive wheels, and regenerative charging is not allowed when it is detected that the state of charge of the battery exceeds 100% of the battery capacity.
In contrast, the use of an electric double layer capacitor in place of a battery has also been considered. This is because electric double layer capacitors have the advantage that they can output a large amount of energy per unit time, that is, electric double layer capacitors have a high energy output density. The high energy out density can respond to a high electric demand such as a high peak electric power at the time of rapid acceleration.
However, when an electric double layer capacitor is used instead of a battery, the following problem arises. That is, there is the possibility that the performance of the electric double layer capacitor will be degraded when a high voltage, which is higher than the withstand voltage of the electric double layer capacitor, is applied between both terminals of the capacitor. Accordingly, it is necessary to restrict the voltage applied between both terminals of the electric double layer capacitor.
However, when the regenerative charging is controlled based on the state of charge of the capacitor, a problem arises in the case of the electric double layer capacitor. This is because the state of charge and the terminal voltage between both terminals of the electric double layer capacitor are not directly related.
Since the internal resistance of the electric double layer capacitor is larger than that of a battery and since a voltage is generated due to the internal resistance when a current flows into the electric double layer capacitor due to regenerative charging, a higher voltage than the voltage determined by the state of charge is applied between both terminals of the capacitor.
Therefore, degradation of the performance or degradation of the insulation of the electric double layer capacitor can not be prevented using the voltage value obtained by the state of charge.
The present invention has been made to solve above problems. It is an objective of the present invention to provide a control apparatus for hybrid cars, capable of avoiding degradation of the insulation or the performance of an electric double layer capacitor.
According to the first aspect of the present invention, a control apparatus is provided in a hybrid car comprising: an engine for outputting a driving force of a car; a clutch connected with an output shaft of said engine including a pair of engaging elements (the engaging elements 11 and 12 in the present embodiment) for connecting or disconnecting the input side and the output side of the shaft for transmitting the driving power; driving wheels (the driving wheels W in the present embodiment) connected to the output of the clutch; a motor (the main motor 19 in the present embodiment) connected to the driving wheels for outputting the driving force of the car and can also be actuated as a regenerative brake; an electric double layer capacitor (the electric double layer capacitor 21 in the present embodiment) for supplying electric power to said motor when the motor outputs the driving power, and for storing the electric power generated by said motor when said motor is actuated as a regenerative brake; a switching device (the power drive unit 23 in the present embodiment) for connecting or disconnecting said electric double layer capacitor and said motor; a voltage detecting device for detecting a terminal voltage (the terminal voltage Vtotal in the present embodiment) of said electric double layer capacitor; and a control device (the control circuit 20 in the present embodiment) for controlling said clutch and said switching device in response to the terminal voltage detected by said voltage detecting device; wherein, during the time that the motor is actuated as a regenerative brake, when the terminal voltage detected by said voltage detecting device exceeds a predetermined standard value, said control device disconnects the connection between the electric double layer capacitor and the motor by said switching device and connects said pair of engaging elements included in said clutch.
According to the above embodiment, the terminal voltage of the electric double layer capacity is detected by the voltage detecting device, the thus detected voltage is compared with the standard value corresponding to the withstand voltage of the electric double layer capacitor, and if the detected voltage exceeds the standard value, the connection between the electric double layer capacitor and the motor is disconnected by the switching device, so that the electric double layer capacitor is protected from being subjected to a voltage higher than the withstand voltage of the electric double layer capacitor.
According to the second aspect of the present invention, a control apparatus of a hybrid car is provided according to the first aspect, wherein said voltage detecting device detects terminal voltages (terminal voltages V1, V2, . . . , Vn in the present embodiment) of respective individual cells (cells 211, 212, . . . , 21n in the present embodiment) included in said electric double layer capacitor.
According to the above structure, the voltage detecting devices detects the terminal voltages of individual cells and charging of the electric double layer capacitor is controlled based on these detected voltages, so that the individual cells included in the electric double layer capacitor are protected from being subjected to a voltage higher than the withstand voltage of each cell.